@Warwick - Good description on what you did but the key information is missing. Key information is how you set the UK English default languages and how you saved the template and how you set it as your default template.
I made a test im my LibO4.3.4.1 on XP, Default language is English US, one of the other installed languages is German, which I used for the test.
1 - Open a new Writer document
2 - Typed a sentence in German so that LibO could recognize that this is German
3 - Tools > Language > For all text > German - All words were indicated as correct except for the few I mistyped on purpose. I used right click to get the correct spelling. Thus, the language setting are correct for this document
4 - File > Save as template using a new name for this template but I did not set this template as my default template.
If I want to use this German language template as my default template, I need to flag it as my default template. Otherwise my previous default template opens automatically whenever I open a new Writer file.
For setting your template as your default template you can do:
File > Template Menu > Save as template (even if you don’t want to save the template again) > select the template you want to use your default template > click Set as default (in the top of the window)
I recommend you to also look into the guides, which you can download free of charge from: https://wiki.documentfoundation.org/Documentation/Publications